The present invention relates to a method of treating an exhaust gas discharged from a nitric acid plant. More particularly, it relates to an improvement in a method of catalytically reducing nitrogen oxides (hereinafter referred to as NO.sub.x) in the compressed exhaust gas discharged from the compressed absorption type nitric acid plant.
The exhaust gas which is described from the compressed absorption type nitric acid plant has a pressure of 1-15 Kg/cm.sup.2 G and usually is fed to the exhaust gas turbine after it is heated by heat-exchange or exhaust gas combustion etc. whereby the energy of the exhaust gas is utilized.
In these methods of removing NO.sub.x from the exhaust gas, it is advantageous to perform the catalytic reduction of NO.sub.x by reacting NO.sub.x with ammonia in the presence of a catalyst at high temperatures and high pressures to give smaller volumes and higher conversions. It has been recommended to provide an apparatus for removing NO.sub.x in a gas flow at the upper stream of the exhaust gas turbine.
In the conventional methods, it is possible to conduct the catalytic reduction of NO.sub.x in the exhaust gas in high efficiency. However, since ammonia is used as the reducing agent, the formation of ammonium nitrate by the acid-base reaction of ammonia with the residual NO.sub.x can not be prevented. Under some conditions, ammonium nitrate is deposited in the system.
The deposition of ammonium nitrate results clogging of the apparatus, pipes, valves, etc. When the solid ammonium nitrate is fed into the high precision and high speed exhaust gas turbine, it is believed that damage is done to the exhaust gas turbine and that explosions can occur which can cause serious disasters.
The gas flow which passes through the exhaust gas turbine has low temperature so that ammonium nitrate easily deposits thereby resulting in clogging of the system. Accordingly, it is necessary to prevent the deposition of ammonium nitrate in these systems.